1. Field of Invention
The invention relates to Light Management Systems (LMSs). The invention is more particularly related to improvements to LMS and their applications to reflective microdisplay based video projectors.
2. Discussion of Background
Light Management Systems (LMSs) are utilized in optical devices, particularly projection video devices and generally comprises a light source, condenser, kernel, projection lens, and a display screen, and related electronics. The function of the components of a video projector 100 is explained with reference to FIG. 1. As shown, white light 110 is generated by a light source 105. The light is collected, homogenized and formed into the proper shape by a condenser 115. UV and IR components are eliminated by filters (e.g., hot/cold mirrors 116/117). The white light 110 then enters a prism assembly 150 where it is polarized and broken into red, green and blue polarized light beams. A set of reflective microdisplays 152A, 152B, and 152C are provided and positioned to correspond to each of the polarized light beams (the prism assembly 150 with the attached microdisplays is called a kernel). The beams then follow different paths within the prism assembly 150 such that each beam is directed to a specific reflective microdisplay. The microdisplay that interacts with (reflects) the green beam displays the green content of a full color video image. The reflected green beam then contains the green content of the full color video image. Similarly for the blue and red microdisplays. On a pixel by pixel basis, the microdisplays modulate and then reflect the colored light beams. The prism assembly 150 then recombines the modulated beams into a modulated white light beam 160 that contains the full color video image. The resultant modulated white light beam 160 then exits the prism assembly 150 and enters a projection lens 165. Finally, the image-containing beam (white light beam 160 has been modulated and now contains the full color image) is projected onto a screen 170.
Commercially available prism assemblies include:    Digital Reflection's Star Prism    Philip's Trichroic Prism    IBM's X Prism with 3 PBS    S-Vision/Aurora System' Off-Axis Prism    Digital Reflection's MG Prism    ColorLink's ColorQuad Prism    Unaxis' ColorCorner Prism
In the prism assembly, pathlengths are precisely matched. That is, the optical distance [] from each of the three microdisplays to an exit face (or output face) 155 of the prism assembly is essentially identical. This allows the microdisplays to be simultaneously in focus at the projection lens. In most currently available prism assemblies, the configuration of the prism assembly consists of precisely formed optical components that have been bonded together. The specific construction techniques by which this is accomplished provides differing advantages and disadvantages.
In some prism assembly configurations, an air gap is introduced between the microdisplays and a face on the prism assembly where the microdisplays are mounted. The air gap is a legitimate approach to accomplish pathlength matching, but has substantial disadvantages. For example, anti-reflection (AR) coatings are needed on the outer surfaces of the microdisplays and the prism assembly faces. The three microdisplays are aligned with respect to each other along all 6 axes of the microdisplay (x, y, z, roll, pitch, and yaw). Alignment is generally performed using mechanical positioners. Once alignment has been accomplished, the problem of maintaining the required precise alignment during the mechanical shock of appliance transport and during the thermal expansion/contraction that occurs while the video projector is in use still remains. In addition, the AR surfaces are exposed to dust, moisture and other atmospheric contaminates that may cause them to degrade. All of these factors reduce video projector performance.
In other prism assembly configurations the microdisplays are bonded to the faces of the prism assembly. Pathlength matching is accomplished by making the prism assembly have “perfect” (very precise) dimensions. Technologies currently being considered for producing these “perfect” dimensions include:
1. Tight Tolerance Component Fabrication
Source components may be fabricated to an extremely tight tolerance. However, such components are not currently available in high volume from vendors within the optics industry. When available, they will be very expensive.
2. Sort Components By Size
Measuring each component in an inventory and matching similarly sized components. The matched components are then used to construct a prism assembly. However, this requires an increased inventory of components from which to select matched sets of components.
3. Utilize Automated Assembly Equipment
The equipment measures the dimensions of each optical component and then actively adjusts their position during the assembly process. Such equipment must be custom designed and is expected to be quite expensive and inflexible.
In all three cases, extremely tight tolerances must be applied to the process used to assemble the optical components into the prism assembly. In all three cases, the outside dimensions of the resulting prism assembly, although having matched pathlengths, can still fall within a wide range. This requires that provisions be made within the video projector to mechanically adjust the position of the prism assembly with respect to the projection lens. Although bonding the microdisplays makes fabrication of the prism assembly more difficult, it has the advantage of eliminating the possibility of eventual misalignment of the microdisplays. In addition, the monolithic construction eliminates exposed surfaces and possible modes of degradation.
The prism assembly configurations each include several different types of plastic and/or glass materials. These disparate materials are bonded together. However, a difficulty arises because each material will have a different coefficient of thermal expansion. Since the prism assembly and its components will inevitably heat and cool during operation, the resulting expansion/contraction of the materials will generate stress (in fact, the process of assembly itself can build mechanical stresses into the prism assembly). Mechanical stress generates optical birefringence. Birefringence effects the polarization of the light beams traveling through the prism assembly and can be visualized on the screen as an undesirable artifact. It is, therefore, important to minimize the occurrence of stress within the prism assembly. One approach to minimize stress is to utilize glass that, in addition to meeting a long list of optical requirements, also has the lowest possible coefficient of stress induced birefringence. An example of one such glass is Schott's SF-57. The use of such a glass improves the situation but does not eliminate the problem.
Based on the considerations discussed above, it should be understood that there are many benefits to mounting the microdisplays directly onto the faces of the prism assembly. However, other various difficulties arise, including the expense of accomplishing the matching of the pathlengths and preparing microdisplays suitable for direct mounting. Furthermore, manufacturers of LMSs have had difficulties with attempts to implement such approaches in high volume manufacturing of any prism assembly configurations. The invention disclosed in this document consists of a prism assembly and construction techniques that can be applied to the construction of most prism assembly configurations (including all of those listed above). It enables inexpensive, high volume manufacturing of pathlength matched prism assemblies allowing the benefits of subsequent attachment of the microdisplays directly onto the faces of the prism assembly.